Enterprise Resource Planning (ERP) is a software solution which offers the opportunity to strengthen the company's effectiveness, integrating the business processes across functional areas. In order to obtain the expected benefit, a successful implementation of the ERP project is crucial.  The aim of this thesis is to understand which different leadership styles can be applied during the ERP implementation phase, how they influence the overall project outcome and what leadership style can lead to the successful overall project outcome. The research covers following questions. To begin we will study ​which different leadership styles are applied in ERP implementation projects and study the leader’s trait & behavior through the project implementation process. ​Afterwards we will study how significant the leadership style is to achieve a successful overall project outcome and which style better suits the general context of an ERP implementation project.  Survey questions were prepared either to project managers with experience in ERP implementation projects within their company or managers working for consultancy service firms. Concepts from multi-grounded theory i.e. open-, axial- and selective coding for empirical grounding are used as a methodological approach in the thesis. After the empirical and theoretical analysis, the conclusion we could draw was that the task structure and demand for the given situation, in which different type of team members are involved, is determining the factors to achieve the leadership effectiveness and consequently it is considered to be the major element determining the leadership style to be applied during the project.